A substation has a function of supplying power to consumers and includes a protection system that cuts off the supply of power to a position where a problem, such as an electric leakage, a ground fault, or the failure of a device, occurs immediately after the problem occurs. In order to detect the problem, a plurality of protection relays provided in the substation frequently measures physical amounts, such as the current and voltage value of electricity through a power line, at a plurality of points. In the typical substation, the protection relays are provided in a building and are connected to the measurement points of the power lines laid outside the building by a plurality of analog cables. Therefore, a large number of cables are laid at a long distance in the substation.
A transmission line protection system has been proposed which uses a digital network to reduce the number of cables in the substation. In the transmission line protection system, a measuring device (a merging unit or brick) is provided close to the power line, digitalizes physical amount input through a plurality of analog cables, and transmits the digital data to a protection relay through a small number of cables of a digital network, such as Ethernet (registered trademark). The protection system using the digital network can reduce the number of cables.